Use `file` for file name input

[GegznaV]

In read_*() functions, we currently use file, files, filename or con to indicate the path to file or connection. We should use file consistently. Update argument name in these functions:

• read_ENVI
• read_ENVI_HySpex
• read_ENVI_Nicolet

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• ⛔ Issue Enable and fix unit tests for ENVI #5 should be solved first and unit tests must work properly.